1. Field of the Invention
This invention relates to a linear motor in which a movable element shifts linearly.
2. Description of Prior Art
FIGS. 1A-1D show in diagrammatic form a prior art linear motor. In the parts of FIG. 1, a guiding member 1 is provided which is composed of magnetic material, and the movable element Z faces the guiding member across a minute gap to form a three phase linear motor here. Movable element 2 is composed of a pair of iron cores 21 and 22 with three field cores respectively, 211, 212, 213, 221, 222 and 223, permanent magnet 20 which is positioned between the pair of iron cores 21 and 22 to provide bias magnetic flux to said minute gap and three coils 31, 32 and 33 which are wound around adjacent field cores 211 and 221, 212 and 222, and 213 and 223 of the pair of iron cores 21 and 22. Three magnetic teeth a, b and c which have the same pitch as pitch P for magnetic teeth on guiding member 1 are provided on the ends of field cores 211, 212, 213 and 221, 222 and 223 which face corresponding rows of teeth 11 and 12 on guiding member 1. The positions of the teeth on the respective field cores 211, 212 and 213 (221, 222 and 223) which are on the same iron core are shifted from the teeth on adjacent field core by (N.+-.n/m) P where N is an integer, m is the number of phases of the linear motor, n is an integer, m&gt;n, and which is (5+1/3)P in this example). As shown in FIG. 1 (C), magnetic teeth on adjacent field cores of iron cores 21 and 22 have the same phase positions. As shown in FIG. 1 (D), guiding member 1 has rows of magnetic teeth 11 and 12 which are at the same pitch P as magnetic teeth on iron cores 21 and 22 of movable element 2. Rows of magnetic teeth 11 and 12 are shifted as much as 1/2P in the direction of movement of said movable element 2. In a device which has an above construction, said permanent magnet 20 is magnetized in the direction as shown in FIG. 1 (B) and magnetic flux generated by said permanent magnet 20 returns to permanent magnet 20 through field cores 211, 212 and 213 of iron core 21, the row of magnetic teeth 11 which faces magnetic teeth a, b and c across a minute gap, the row of magnetic teeth 12, across the minute gap, the magnetic teeth on each field core 221, 222 and 223 of core 22, and the respective field cores 221, 222 and 223. That means there is bias magnetic flux in the gap between the teeth on said movable element 2 and the teeth on and guiding member 1. When exciting current is line overlapped with said bias magnetic flux.
When exciting current is conducted to coil 31 in the direction in which magnetic flux between field core 211 on the iron core 21 side and guiding member 1 is increased, magnetic flux between field core 221 on the iron core 22 side and guiding member 1 is reduced. As a result, field core 21 on movable element 2 is pulled along member 1 toward the position shown in FIG. 1A in which the gap reactance is minimized.
Then, when exciting current is conducted to coil 32 in the direction in which magnetic flux between field core 212 on the iron core 21 side and guiding path 1 is increased, field core 212 is pulled along guiding member 1 and thus movable element 2 shifts to the right. Movable element 2 shifts as much as 1/3P, as the magnetic teeth a, b and c on field core 212 shift as much as 1/3P. In the same manner, when exciting current is conducted to coils 33, 31, 32, 33, . . . respectively, the moveable element shifts to the right respectively with a minimum shifting length of 1/3P. When exciting current is conducted to coils 33, 32 and 31 respectively, movable element 2 shifts to the left.
On the other hand, if the direction of exciting current conducted to coils 31, 32 and 33 is reversed from said case, magnetic flux between field core 221 and guiding member 1 is increased and movable element is pulled along guiding member 1. Therefore, when exciting current is respectively conducted to coil 32, 33, 31 . . . , in the reverse direction from said case, movable element 2 shifts to the right with 1/3P as a minimum shifting length. Also, when current is conducted to coils 33, 32, 31, . . . respectively in the reverse direction from said case, movable element 2 shifts to the left with 1/3P as a minimum a minimum shifting length. As rows of magnetic teeth 11 and 12 on guiding element 1 have the teeth shifted as much as 1/2P, when the direction of exciting current conducted to the coils is positive and negative (reversed stopping position of movable element 2 are not overlapped with each other and are shifted as much as 1/6P.) That is, movable element 2 shifts as much as a minimum shifting length 1/6P by switching exciting current to be conducted to coils 21, 32 and 33 and changing the direction of exciting current to energize the coils respectively. Thus explanation is given only in terms of single phase energizing, but thrust force can be increased by providing polyphase energizing.
To improve the thrust force of this type of linear motor, it is very effective to increase bias magnetic flux between magnetic teeth on the movable element iron cores and those on the guiding element. One means to expand bias magnetic flux is to increase the total magnetic flux generated by the permanent magnet which provides bias magnetic flux. Another mean is try to minimize leakage flux which does not contribute to the linear motor thrust force by the magnetic flux generated by the permanent magnet.
In the prior art motor which has the construction as shown in FIG. 1 small distance W.sub.1 between adjacent field cores 211 and 221, 212 and 222, and 213 and 223 of iron cores 21 and 22 causes increased leakage flux between them and it is the largest proportion of leakage flux in the entire magnetic circuit. Therefore, the magnetic flux of the permanent magnet is not used effectively and there is a lot of waste, and thus the motor's operation is not very efficient.
If said distance W.sub.1 is increased, it is possible to reduce leakage magnetic flux, but width W.sub.2 of guiding member 1 must also be expanded accordingly. Expansion of width W.sub.2 of guiding member 1 must be along the entire length of guiding member 1, and the weight of the guiding member increases greatly. Due to this increase, the weight of the device into which the motor is integrated naturally increases. Also, for an X-Y plotter into which the motor is integrated, the weight increase of the guiding member causes an adverse effect on the velocity of plotting, because the motor of one spindle carries out shift positioning of the motor of another spindle with its guiding member. Movable element magnetic teeth of the prior art motor as shown in FIG. 1 are difficult to machined precisely to an accurate dimension. Generally, movable element magnetic teeth are machined by feeding a machining knife at the magnetic teeth pitch. When the number of linear motor phases is increased to 5, 6, . . . , the magnetic teeth for the different phases on the same iron core is increased to 5, 6, . . . and their pitch differences become 2.pi./5 and 2.pi./6, . . . , so that the feeding pitch of the machining knife must be changed many times during machining, and therefore the machining cannot be carried out precisely.
As a result the linear motor is difficult to make with precision. Also, the thrust force for shifting generated in each field core is actually very uneven in this type of linear motor. The biggest reason for this is that the amount of bias magnetic flux which passes each field core is very different. This difference stems from magnetic non-contrast and most of it is caused by leakage flux. FIG. 2 is a perspective view of movable element 2 to explain leakage magnetic flux of the prior art motor shown in FIG. 1. Half cylinder parts 81 and 82 shown in dotted lines show the leakage flux path in which bias magnetic flux is leaked on both sides of a pair of iron cores 21 and 22. This kind of leakage path is for the level of bias magnetic flux which passes through each field core. That is, compared with bias magnetic flux which passes through central field cores 212 and 222, bias magnetic flux which passes through field cores 211, 221, 213 and 223 which are positioned outside of 212 and 222 is smaller.
In addition to that, the magnetic flux of the coils are different depending on each field core. This is mainly due to the difference of magnetic resistance depending on the length of the iron core magnetic path. For instance, in the case of iron core 21, magnetic flux generated by coil 32 which is wound around field core 212 at the center of iron core 21 separately goes around field core 211 and 213 on the both sides of coil 32, but magnetic flux, for instance, of coil 31 wound around outside field core 211, goes around field core 212 which is next to 211, and, further, field core 213 which is next to 212. Thus, in the latter case, the magnetic path is slightly longer and the magnetic flux in the outside field cores tends to be slightly smaller than that of the central field cores. It is becoming apparent that deviation of thrust force caused by each field core is mainly affected by two kinds of deviation, deviation of bias magnetic flux and magnetic flux caused by coils. Deviation of magnetic flux caused by coils can be made almost negligible by lowering the iron core reluctance, but deviation of bias magnetic flux is relatively large and, as a consequence, the linear motor thrust force of the prior art motor deviates to a large extent.